Heparin, thrombin and Factor Xa inhibitors

Direct and indirect coagulation inhibitors are used to inhibit the activity of the serine proteases of the coagulation system. Indirect inhibitors act via antithrombin and heparin cofactor II. The main representatives are heparins, lowmolecular-weight heparins, fondaparinux, idraparinux and danaparoid. They bind to antithrombin and potentiate the inactivation of factor Xa and other serine proteases. Direct thrombin inhibitors bind reversibly to thrombin without cofactor. Anticoagulants are determined by global and specific anticoagulant methods. New anticoagulants are developed such as oral factor Xa inhibitors, oral thrombin inhibitors, antibody against activated factor VII, recombinant tissue pathway inhibitor to improve inhibition of blood coagulation or to induce nonanticoagulant effects (e. g. activated protein C in septicaemia). New anticoagulant methods are developed to improve and specify the anticoagulant effect of anticoagulants in thromboembolic diseases.     

New anticoagulants

The limitations of heparin and warfarin have prompted the development of new anticoagulant drugs for prevention and treatment of venous and arterial thromboembolism. Novel parenteral agents include synthetic analogs of the pentasaccharide sequence of heparin that mediates its interaction with antithrombin. Fondaparinux, the first synthetic pentasaccharide, is licensed for prevention of venous thromboembolism (VTE) after major orthopedic surgery and for initial treatment of patients with VTE. Idraparinux, a long-acting pentasaccharide that is administered subcutaneously once-weekly, is being compared with warfarin for treatment of VTE and for prevention of cardioembolic events in patients with atrial fibrillation. New oral anticoagulants include direct inhibitors of thrombin, factor Xa and factor IXa. Designed to provide more streamlined anticoagulation than warfarin, these agents can be given without routine coagulation monitoring. Ximelagatran, the first oral direct thrombin inhibitor, is as effective and safe as warfarin for prevention of cardioembolic events in patients with atrial fibrillation. However, ximelagatran produces a three-fold elevation in alanine transaminase levels in 7.9% of patients treated for more than a month, the long-term significance of which is uncertain. Whether other direct thrombin inhibitors or inhibitors of factors Xa or IXa also have this problem is under investigation. After a brief review of coagulation pathways, this paper focuses on new anticoagulants in advanced stages of clinical testing.     

Heparin mimetics with anticoagulant activity

Heparin, a sulfated polysaccharide belonging to the glycosaminoglycan family, has been widely used as an anticoagulant drug for decades and remains the most commonly used parenteral anticoagulant in adults and children. However, heparin has important clinical limitations and is derived from animal sources which pose significant safety and supply problems. The ever growing shortage of the raw material for heparin manufacturing may become a very significant issue in the future. These global limitations have prompted much research, especially following the recent well‐publicized contamination scandal, into the development of alternative anticoagulants derived from non‐animal and/or totally synthetic sources that mimic the structural features and properties of heparin. Such compounds, termed heparin mimetics, are also needed as anticoagulant materials for use in biomedical applications (e.g., stents, grafts, implants etc.). This review encompasses the development of heparin mimetics of various structural classes, including synthetic polymers and non‐carbohydrate small molecules as well as sulfated oligo‐ and polysaccharides, and fondaparinux derivatives and conjugates, with a focus on developments in the past 10 years.     

Nebulized anticoagulants for acute lung injury - a systematic review of preclinical and clinical investigations

ABSTRACT: BACKGROUND: Data from interventional trials of systemic anticoagulation for sepsis inconsistently suggest beneficial effects in case of acute lung injury (ALI). Severe systemic bleeding due to anticoagulation may have offset the possible positive effects. Nebulization of anticoagulants may allow for improved local biological availability and as such may improve efficacy in the lungs and lower the risk of systemic bleeding complications. METHOD: We performed a systematic review of preclinical studies and clinical trials investigating the efficacy and safety of nebulized anticoagulants in the setting of lung injury in animals and ALI in humans. RESULTS: The efficacy of nebulized activated protein C, antithrombin, heparin and danaparoid has been tested in diverse animal models of direct (for example, pneumonia-, intra-pulmonary lipopolysaccharide (LPS)-, and smoke inhalation-induced lung injury) and indirect lung injury (for example, intravenous LPS- and trauma-induced lung injury). Nebulized anticoagulants were found to have the potential to attenuate pulmonary coagulopathy and frequently also inflammation. Notably, nebulized danaparoid and heparin but not activated protein C and antithrombin, were found to have an effect on systemic coagulation. Clinical trials of nebulized anticoagulants are very limited. Nebulized heparin was found to improve survival of patients with smoke inhalation-induced ALI. In a trial of critically ill patients who needed mechanical ventilation for longer than two days, nebulized heparin was associated with a higher number of ventilator-free days. In line with results from preclinical studies, nebulization of heparin was found to have an effect on systemic coagulation, but without causing systemic bleedings. CONCLUSION: Local anticoagulant therapy through nebulization of anticoagulants attenuates pulmonary coagulopathy and frequently also inflammation in preclinical studies of lung injury. Recent human trials suggest nebulized heparin for ALI to be beneficial and safe, but data are very limited.     

Effects of anticoagulant strategies on activation of inflammation and coagulation

Acute inflammatory events, such as those that occur in sepsis, lead to dysregulation of the coagulation cascade. The hemostatic imbalance in sepsis, characterized by the excessive activation of procoagulant pathways and the impairment of anticoagulant activity, leads to disseminated intravascular coagulation and results in microvascular thrombosis, tissue hypoperfusion and, ultimately, multiple organ failure and death. Furthermore, natural anti-inflammatory mechanisms of the endogenous anticoagulants are diminished by the impaired coagulation. Supportive strategies aiming at inhibiting activation of coagulation and inflammation by treatment with exogenous anticoagulants have been found to be beneficial in experimental and initial clinical studies. This review summarizes the available experimental and clinical data regarding the interaction between coagulation and inflammation, focusing on the two anticoagulants which are in clinical use, antithrombin and activated protein C. Identification of the different biological mechanisms of the two endogenous anticoagulants might help to determine target patient populations as well as to develop new anticoagulant analogs that differ in there respective effects in coagulation and inflammation.     

Effects of anticoagulant strategies on activation of inflammation and coagulation

Acute inflammatory events, such as those that occur in sepsis, lead to dysregulation of the coagulation cascade. The hemostatic imbalance in sepsis, characterized by the excessive activation of procoagulant pathways and the impairment of anticoagulant activity, leads to disseminated intravascular coagulation and results in microvascular thrombosis, tissue hypoperfusion and, ultimately, multiple organ failure and death. Furthermore, natural anti-inflammatory mechanisms of the endogenous anticoagulants are diminished by the impaired coagulation. Supportive strategies aiming at inhibiting activation of coagulation and inflammation by treatment with exogenous anticoagulants have been found to be beneficial in experimental and initial clinical studies. This review summarizes the available experimental and clinical data regarding the interaction between coagulation and inflammation, focusing on the two anticoagulants which are in clinical use, antithrombin and activated protein C. Identification of the different biological mechanisms of the two endogenous anticoagulants might help to determine target patient populations as well as to develop new anticoagulant analogs that differ in there respective effects in coagulation and inflammation.     

Unfractionated heparin and other antithrombin mediated anticoagulants.

Clinical practice over the past decade has evolved to include new agents, LMWH and synthetic polysaccharides, that bind to and enhance the activity of antithrombin similar to UFH. These drugs differ from UFH since their anticoagulant effect consists predominantly, or entirely, of anti-Xa activity. More important, the new drugs have greater predictability with regard to dosing. In clinical studies the new agents have proven as good as or better than UFH with regard to efficacy and toxicity. The synthetic polysaccharide may possess the greatest efficacy, but possibly with increased bleeding risk. However, UFH still has one advantage over these agents, the ability of its anticoagulant effect to undergo essentially complete reversal with an available drug, protamine sulfate. Thus, clinical situations favoring UFH over these newer parenteral agents still exist.     

Practical management of anticoagulants in family medicine after orthopedic surgery

In order to prevent postoperative venous thromboembolism, prophylactic anticoagulant therapy is routinely administered to hospitalized patients after total hip arthroplasty (THA) and total knee arthroplasty (TKA). However, after hospital discharge, anticoagulant therapy is primarily managed by a primary care physician (PCP). The agents traditionally used for this purpose are associated with certain limitations that affect anticoagulation management. The new, fixed-dose oral anticoagulants have predictable pharmacokinetic and pharmacodynamic profiles, no requirement for coagulation monitoring, and a low propensity for food and drug interactions. As a result, they have the potential to simplify and improve postoperative outcomes in patients who have undergone THA or TKA and allow for simpler management of anticoagulation in these patients. This article examines the clinical evidence for benefits of the new oral anticoagulants, discusses caveats regarding their appropriate use, and provides some guidance regarding bleeding management with these agents.     

Current Strategies for the Management of Bleeding Associated with Direct Oral Anticoagulants and a Review of Investigational Reversal Agents

BackgroundThe management of life-threatening bleeding in patients who are receiving direct oral anticoagulants (DOACs) is a serious medical concern.ObjectiveThis review provides a concise, balanced overview of the current and future approaches for reversing the anticoagulation effects of DOACs, particularly factor Xa (FXa) inhibitors.DiscussionThe anticoagulant activity of the direct thrombin inhibitor dabigatran can be reversed by idarucizumab, but until recently, options for the management of major bleeding in patients who were receiving FXa inhibitors were limited to nonspecific strategies, including supplementation of clotting factors with prothrombin complex concentrates (PCCs) or activated PCCs for attenuating anticoagulation effects. They appear as a treatment option in many hospital guidelines despite the lack of approval by the U.S. Food and Drug Administration and the lack of rigorous medical evidence supporting their use in this setting. The development of specific reversal agents may provide improved strategies for the management of bleeding. Andexanet alfa is a modified FXa molecule approved in the United States to reverse the anticoagulant effects of FXa inhibitors (rivaroxaban and apixaban) in patients with life-threatening or uncontrolled bleeding. Ciraparantag is a small-molecule inhibitor of multiple anticoagulants that has been investigated in healthy subjects.ConclusionThe current guidelines for management of DOAC-associated bleeding are being updated to reflect that the reversal agent for rivaroxaban and apixaban is now available. For other FXa inhibitors, in the absence of a reversal agent, nonspecific strategies that include PCCs are recommended. The population of patients anticoagulated with DOACs is growing, and we hope that specific reversal agents will improve the approach to management of major bleeding in this population.     

白树花多糖硫酸酯的抗凝血活性研究

目的研究白树花多糖硫酸酯的抗凝血效果及抗凝血机理,为制备合成高效、安全的多糖硫酸酯抗凝剂提供实验依据。方法将可食用的白树花真菌进行发酵,提取水溶性多糖,采用氯磺酸-吡啶法进行硫酸酯化修饰,获得水溶性的白树花多糖硫酸酯。通过测定APTT、TT、凝血因子活性对白树花硫酸酯的抗凝血效果及抗凝血机理进行评估和探讨。结果对正常人血浆进行的抗凝血实验表明,白树花多糖硫酸酯具有明显的抗凝血活性,在5 mg/L的浓度下即可发挥效果,在10 mg/L浓度时,相当于150 U肝素的抗凝血效果;抗凝血机理研究结果表明,白树花多糖硫酸酯作用于内源凝血系统,主要通过抗凝血酶Ⅲ抑制凝血因子Ⅱa和Xa的活性发挥抗凝血作用,抗凝血机理与肝素完全相同。结论通过上述研究结果,我们认为白树花多糖硫酸酯具备肝素的抗凝血特性。同时由于是可食用真菌多糖的制备产物,在防生物污染方面具备一定优势。因此,白树花多糖硫酸酯可作为理想的肝素替代品进行开发及临床应用。     

Update on perioperative bridging in patients on chronic oral anticoagulation

Oral anticoagulation (OAC) with vitamin K antagonists is commonly used for long-term prevention or treatment of arterial or venous thromboembolism. In the USA alone, approximately 250,000 patients will require temporary interruption of OAC annually. Managing anticoagulation in those patients on chronic OAC who require invasive procedures continues to be a major clinical dilemma. This article summarizes the existing evidence in light of the recommendations of the American College of Chest Physicians. Management of anticoagulation in the perioperative period will continue to be an important clinical challenge and an evolving area of research. If new oral anticoagulants are successful in replacing warfarin, the entire perioperative anticoagulation scene will change.     

Anticoagulation with direct thrombin inhibitors during extracorporeal membrane oxygenation

Use of extracorporeal membrane oxygenation to support patients with critical cardiorespiratory illness is increasing. Systemic anticoagulation is an essential element in the care of extracorporeal membrane oxygenation patients. While unfractionated heparin is the most commonly used agent, unfractionated heparin is associated with several unique complications that can be catastrophic in critically ill patients, including heparin-induced thrombocytopenia and acquired antithrombin deficiency. These complications can result in thrombotic events and subtherapeutic anticoagulation. Direct thrombin inhibitors (DTIs) are emerging as alternative anticoagulants in patients supported by extracorporeal membrane oxygenation. Increasing evidence supports DTIs use as safe and effective in extracorporeal membrane oxygenation patients with and without heparin-induced thrombocytopenia. This review outlines the pharmacology, dosing strategies and available protocols, monitoring parameters, and special use considerations for all available DTIs in extracorporeal membrane oxygenation patients. The advantages and disadvantages of DTIs in extracorporeal membrane oxygenation relative to unfractionated heparin will be described.     

Experimental and clinical results with the thrombin inhibitor Argatroban

Argatroban is the smallest anticoagulant molecule of direct thrombin inhibitors. The main attributes of this synthetic drug are its rapid onset of anti-thrombin action, the rapid reversibility of its anticoagulant effect, potent inhibition of clot-bound thrombin, the absence of antibody formation and no need for dosage adjustment in patients with renal impairment. It is eliminated by hepatic metabolism. These properties make argatroban a predictable anticoagulant with intravenous use in a routine clinical setting. Argatroban is approved in the US and Canada for both prophylaxis and treatment of thrombosis in patients with heparin-induced throm-bocytopenia (HIT) and in the US as an antithrombotic agent during percutaneous coronary interventions in patients with HIT or a history of HIT. Preliminary reports document the feasibility of using argatroban for anticoagulation during hemodialysis and as adjunct to thrombolysis for treatment of myocardial infarction. Current recommendations for argatroban monitoring are to use the aPTT (activated partial thromboplastin time) for low doses and the ACT (activated clotting time) for high doses. The specific inhibition of thrombin can be measured with the ECT (ecarin clotting time).     

透析膜的抗凝药物修饰及体外抗凝性能评价

【目的】对透析膜进行抗凝药物修饰，并初步评价其抗凝性能。【方法】以聚丙烯腈透析膜为基材，采用分子自组装技术进行膜修饰，通过亲水角测量法以及抗凝药物吸光度测定跟踪组装过程；复钙化时间检测、血小板粘附试验评价修饰膜的抗凝性能。【结果】亲水角随分子自组装的交替进行而呈锯齿状规律性的变化，抗凝药物阿加曲班的吸光度随组装的进行基本呈线性增加；修饰膜的复钙化时间较未修饰膜明显延长，血小板粘附数目也明显减少，且活力降低。【结论】抗凝药物膜修饰技术基本可行，所得修饰膜具有较好的抗凝性能。     

Heparin‐induced thrombocytopenia – therapeutic concentrations of danaparoid,unlike fondaparinux and direct thrombin inhibitors,inhibit formation of platelet factor 4–heparin complexes

Summary. Background: Treatment of heparin‐induced thrombocytopenia (HIT), a disorder in which anti‐platelet factor 4 (PF4)–heparin antibodies cause platelet activation and hypercoagulability, requires alternative (non‐heparin) anticoagulation. Treatment options include direct thrombin inhibitors [lepirudin and argatroban (approved), and bivalirudin], danaparoid (approved) (mixture of anticoagulant glycosaminoglycans), or fondaparinux (synthetic heparin‐mimicking pentasaccharide). PF4–heparin complexes form at optimal stoichiometric ratios. Objectives: To compare the effects of these various non‐heparin anticoagulants in disrupting the formation of PF4–heparin complexes, and PF4‐containing immune complexes. Patients/methods: Sera were obtained from patients with serologically confirmed HIT. The effects of the alternative anticoagulants on PF4 and PF4–heparin complex interactions with platelets, as well as HIT antibody binding and platelet activation, were investigated. Results: Danaparoid at very low concentrations increased PF4 binding to platelets. In therapeutic concentrations, however, it decreased PF4 binding to platelets (P = 0.0004), displaced PF4–heparin complexes from platelets (P = 0.0033) and PF4 from the surface of a PF4‐transfected HEK‐293 EBNA cell line expressing the PF4 receptor CXCR3‐B (P = 0.0408), reduced PF4–heparin complex size (P = 0.025), inhibited HIT antibody binding to PF4–heparin complexes (P = 0.001), and prevented platelet activation by HIT antibodies (P = 0.046). Although fondaparinux also interfered with PF4 binding to platelets, HIT antibody binding to PF4–heparin complexes, and activation of platelets by HIT antibodies, these effects occurred only at supratherapeutic concentrations. The direct thrombin inhibitors had no effect at any concentrations. Conclusions: Danaparoid uniquely interferes with the pathogenesis of HIT by disrupting PF4‐containing immune complexes at therapeutic dose concentrations. It is possible that these effects contribute to its therapeutic efficacy.     

Reversal of drug-induced anticoagulation: old solutions and new problems

Anticoagulant drugs are taken by millions of patients throughout the world. Warfarin has been the most widely prescribed anticoagulant for decades. In recent years, new oral anticoagulants have been approved for use, are being positioned as alternatives to warfarin, and represent an enormous market opportunity for pharmaceutical companies. Requests for urgent reversal of anticoagulants are not uncommon especially in the setting of critical bleeding. This review summarizes information on reversal of warfarin by vitamin K, plasma, prothrombin complex concentrates, and recombinant VIIa. In addition, we emphasize the lack of current evidence supporting reversibility of the new oral direct thrombin inhibitors and Factor Xa inhibitors. This review is presented to assist transfusion medicine specialists, hematologists, and other clinicians who prescribe blood components for reversal of drug-induced anticoagulation.     

Anticoagulation by factor Xa inhibitors

Summary. Background: Therapeutic agents that regulate blood coagulation are critical to the management of thrombotic disorders, with the selective targeting of factor (F) Xa emerging as a promising approach. Objective: To assess anticoagulant strategies targeting FXa. Methods: A deterministic computational model of tissue factor (Tf)‐initiated thrombin generation and two empirical experimental systems (a synthetic coagulation proteome reconstruction using purified proteins and a whole blood model) were used to evaluate clinically relevant examples of the two available types of FXa‐directed anticoagulants [an antithrombin (AT)‐dependent agent, fondaparinux, and an AT‐independent inhibitor, Rivaroxaban] in experimental regimens relevant to long‐term (suppression of new Tf‐initiated events) and acute (suppression of ongoing coagulation processes) clinical applications. Results: Computational representations of each anticoagulant’s efficacy in suppressing thrombin generation over a range of anticoagulant concentrations in both anticoagulation regimens were validated by results from corresponding empirical reconstructions and were consistent with those recommended for long‐term and acute clinical applications, respectively. All three model systems suggested that Rivaroxaban would prove more effective in the suppression of an ongoing coagulation process than fondaparinux, reflecting its much higher reactivity toward the prothrombinase complex. Conclusion: The success of fondaparinux in acute settings in vivo is not explained solely by its properties as an FXa inhibitor. We have reported that FIXa contributes to the long‐term capacity of clot‐associated catalysts to restart a coagulation process, suggesting that the enhanced anti‐FIXa activity of fondaparinux‐AT may be critical to its success in acute settings in vivo.     

Laboratory assessment of the anticoagulant effects of the next generation of oral anticoagulants

Summary. In contrast to vitamin K antagonists, which reduce the functional levels of several coagulation factors, the new oral anticoagulants specifically target either thrombin or factor Xa. These new agents have such predictable pharmacokinetics and pharmacodynamics that routine coagulation monitoring is unnecessary. However, there are still some situations in which measurement of anticoagulant effect may be required. The coagulation assays that are used to monitor heparin derivatives or vitamin K antagonists may not always accurately reflect the anticoagulant activity of the new oral anticoagulants, and specialized assays may be needed. In this article, we: (i) identify situations in which assessment of anticoagulant effect may aid treatment decisions; (ii) describe the effects of the new oral anticoagulants on the various coagulation tests; (iii) review the specialized coagulation assays that have been developed to measure the anticoagulant effects of the new oral anticoagulants; and (iv) provide a clinical perspective on the role of coagulation testing in the clinical management of patients treated with the new oral anticoagulants.     

Diagnosis and management of heparin-induced thrombocytopenia

Heparin use is ubiquitous, wherein 1 to 5% of patients exposed to standard unfractionated heparin develop thrombocytopenia due to antibodies to a complex of heparin and platelet factor 4. Classic features include onset of thrombocytopenia after 5 to 10 days of ongoing heparin exposure, a 50% fall in the platelet count from baseline, resolution of the thrombocytopenia 5 to 10 days after cessation of heparin and a high risk of thrombosis noted in 30 to 75% of patients with heparin-induced thrombocytopenia (HIT) in terms of every-other-day platelet-count monitoring in patients on standard unfractionated heparin. And those patients developing thrombocytopenia necessitate an accurate, readily accessible diagnostic test for HIT. Diagnosis has been recently facilitated by the development of an enzyme-linked immunsorbent assay (ELISA) test for the heparin–P4 antibody complex, although this test carries a relatively low specificity. Widespread use of the ELISA demonstrates a relatively high prevalence of the antibody in patients exposed to heparin in certain settings, such as cardiopulmonary bypass, wherein a quarter of patients have a positive ELISA of unclear significance. Once HIT is diagnosed, the high risk of thrombosis necessitates empiric anticoagulation with an antithrombin such as argatroban or lepirudin, or the heparinoid danaparoid. Additional agents under further study include the antithrombin bivalirudin and the pentasaccharide fondaparinux. Future issues in HIT include increasing awareness for HIT, improving the specificity of HIT testing and the development of new anticoagulants for HIT that will enable out-patient management.     

Polyphosphate as a general procoagulant agent

Summary. Background: Polyphosphate is secreted by activated platelets and we recently showed that it accelerates blood clotting, chiefly by triggering the contact pathway and promoting factor (F) V activation. Results: We now report that polyphosphate significantly shortened the clotting time of plasmas from patients with hemophilia A and B and that its procoagulant effect was additive to that of recombinant FVIIa. Polyphosphate also significantly shortened the clotting time of normal plasmas containing a variety of anticoagulant drugs, including unfractionated heparin, enoxaparin (a low molecular weight heparin), argatroban (a direct thrombin inhibitor) and rivaroxaban (a direct FXa inhibitor). Thromboelastography revealed that polyphosphate normalized the clotting dynamics of whole blood containing these anticoagulants, as indicated by changes in clot time, clot formation time, alpha angle, and maximum clot firmness. Experiments in which preformed FVa was added to plasma support the notion that polyphosphate antagonizes the anticoagulant effect of these drugs via accelerating FV activation. Polyphosphate also shortened the clotting times of plasmas from warfarin patients. Conclusion: These results suggest that polyphosphate may have utility in reversing anticoagulation and in treating bleeding episodes in patients with hemophilia.